The present invention relates to a method and a device for temperature-dependent control of an electric motor, comprising an electric motor which derives electric energy from an energy source for outputting mechanical power, a control device for adjusting the power output of the electric motor and means for detecting the temperature of a component of the electric motor, wherein at a temperature of the component above a temperature threshold the control device reduces the power of the electric motor.
Such methods and devices as for example disclosed in DE 102 03 051 A1 or DE 10 2005 050 741 A1 are used in electric motors for protecting components against overheating. On one hand the electric motor can heat up in a hot environment (external heating) and on the other hand during the conversion of electric and mechanical energy (self-heating). So far the conventional approaches for avoiding heat-related damage provide for temporarily reducing the power of the electric motor (so-called derating). However, these approaches can only counteract the cause for the self-heating. Therefore such methods and devices are only used in temperature ranges, which the electric motor can only reach due to self-heating, wherein this temperature range is so high that it cannot be reached by external heating. The electric motors have to be constructed correspondingly robust with regard to heat damage. This applies particularly to the used magnets, which have to be provided with very expensive additives from the group of rare earths for protection against demagnetization resulting from high temperatures.
The category defining DE 10 2005 052 745 A1 discloses a system and method for estimating the motor temperature for protection against overuse of the motor, wherein an estimated temperature of the motor is determined based on the ambient temperature, a current supplied to the motor, a time during which the current is supplied to the motor and at least one thermal property. The estimated motor temperature is used for reducing a voltage supplied to the motor when the estimated motor temperature is greater than a safe operating temperature of the motor. The voltage is increased again when the estimated motor temperature returns to a degree that is not greater than the safe operating temperature of the motor. A disadvantage is that also in this case the power reduction is not adjusted to the actual cause of the increase of the motor temperature. As a consequence the power of the motor may be reduced unnecessarily in a hot environment in addition the components, in particular the magnets, still have to be constructed expensively to withstand high temperatures.